1. Field of the Invention
The present invention relates in general to a pulverized coal flow control system, and more particularly, to a new and useful flow control system for accurately controlling the mass flow rate of pneumatically transported pulverized coal.
2. Description of the Related Art
In a pulverized coal burning boiler, one or more pulverizers are used to grind lumps of crushed coal into particulates with a certain desired size distribution. The airborne pulverized coal (PC) is typically transported to each burner in pipes ranging from 8-24 inches in diameter. The number of burners fed by one pulverizer can be anywhere from 2-13 supplied by as many pipes carrying the pulverized coal.
In a multiple-burner boiler operation, it is highly desirable to maintain a good balance among all the burners to attain high thermal efficiency and to maintain close control of stack emissions. The flow of pulverized coal is the single most important process variable that needs to be controlled to achieve balanced operation among several burners. Balanced burner operation requires that the mass flow rate of both air and pulverized coal be the same among all the pipes leading to the burner within certain operating limits. Each supply pipe or feed pipe installed between the pulverizer and the burner generally has a hydraulic resistance which is somewhat different from the other supply pipes or feed pipe lines due to differences in overall length of each pipe line, and the type and number of bends used for each pipe line. These variations in line resistance can cause an imbalance of the primary air and coal flow among the pulverized coal supply pipe lines. This possible imbalance must be corrected to insure efficient combustion.
Common industry practice is to add a fixed resistance orifice or sections of small diameter pipe in the line that has a lower resistance than desired as is illustrated by 8 and 9 in FIG. 1. Then the balancing of primary air flow in each line is confirmed by measuring the air flow in each line with a pitot tube in the absence of pulverized coal flow. However, the balanced primary air flow alone in the above situation does not necessarily insure a balanced pulverized coal flow in the system due to the asymmetric flow distribution at the pulverizer outlet and the peculiarities in the airborne solids flow. Plant operators have reported in excess of 10% deviation in pulverized coal flow from the average in a system that had been balanced for primary air flow using fixed resistance orifices and pipes.
While there are a number of pulverized coal flow meters at various stages of development in the industry, there has not been any commercially available flow control system for pulverized coal transport lines. The primary reason for the absence of such a system is because it is very difficult to design a reliable control element that can meet a set of very tough operational requirements, namely:
1. For long-term, reliable service, the control element must be highly erosion-resistant if it is to be exposed to flowing coal particles. The velocity of primary air in feed pipes is around 70 ft/sec at full load but can be as high as 100 ft/sec, and as low as 40 ft/sec at partial load. Pulverized coal moving at these high velocities is an extremely abrasive flowing medium that is capable of eroding away any common metallic intrusion in its stream;
2. The element must not appreciably increase the pressure drop of the line; the maximum tolerable increase in pressure drop would vary from plant to plant but the permissible increase is generally very small;
3. The element should not interfere with the normal flow of the primary air keeping the pulverized coal particles airborne when the controlling function is not needed;
4. The control should be sensitive enough to effect slight changes as small as 1-2% in pulverized coal mass flow rate; and
5. The control system needs to be readily retrofitable to existing plants at reasonable installation and operation cost.
U.S. Plant application Ser. No. 07/197,926 filed May 24, 1988 titled "Pulverized Coal Flow Control System" which is also owned by the present Assignee is a continuation of application Ser. No. 07/106,830 which was filed Oct. 6, 1987 and issued as U.S. Pat. No. 4,830,287 on May 16, 1989. This reference discloses a pulverized coal flow control system with an aspirator connected to the outer wall of a bend provided in the supply pipe. The aspirator draws off an amount of mixture from the supply pipe and re-injects it back into the pulverizer. As a result, the flow of mixture is controlled through the supply pipe.
The foregoing pulverized coal flow control system needs diverted control bypass lines which can be costly to install. The present invention provides for an arrangement that is totally free of these costly bypass lines which makes it simpler to install and operate while further eliminating the possibility of any pluggage in the bypass lines.